Recovery of alkalies and terephthalic acid from aqueous solutions containing alkali salts of terephthalic acid



United States Patent RECOVERY' OF ALKALIES AND TEREPHTHALIC ACID. FROM AQUEOUS SOLUTIONS CONTAIN- ING ALKALI SALTS 0F TEREPHTHALIC ACID Hartwig Schiitt, Hagen, Westphalia, Germany, assignor to Henkel & Cie. G.m.h.H., Dusseldorf-Holthausen, Germany, a corporation of'Germany No Drawing. Application June 11, 1956 Serial No. 590,382

Claims priority, application Germany June 13, 1955 7 Claims. (Cl. 260-525) This invention relates to a novel method of separating terephthalic acid from aqueous solutions comprising substantial quantities of alkali salts of terephthalic acid, and more particularly to a method of recovering terephthalic acid fromsuch solutions with the aid of carbon dioxide.

In the production of alkali salts of terephthalic acid involving the heating of alkali salts of orthophthalic, isophthalicor benzoic acid in an inert atmosphere, preferably in an atmosphere of carbon dioxide, a solid reaction product is obtained which contains substantial quantities of alkali terephthalates. Terephthalic acid is recovered from this reaction product by dissolving the product in water, acidifying the aqueous solution with an acid which is stronger than terephthalic acid, especially with hydrochloric acid or sulfuric acid, whereby the alkali terephthalates are transformed into free terephthalic acid which precipitates from the acidified solution in very pure form, and separating the precipitate from the solution. Such processes are described in co-pending applications Serial No. 392,512, filed November 16, 1953, now abandoned, and Serial No. 395,609, filed December 1, 1953, now abandoned. In a similar fashion, terephthalic acid may also be recovered from reaction mixtures containing substantial amounts of alkali terephthalates obtained from other sources; for example, the above procedure is also employed to separate terephthalic acid from the reaction mass obtained in the alkaline hydrolysis of synthetic fiber waste material containing terephthalic acid derivatives, or from the wash solutions obtained, by washing terephthalic acid out of the oxidation products of suit able organic compounds, such as benzene derivatives alkyl-substituted in the paraposition, with alkalies.

However, this method of recovering terephthalic acid has the serious disadvantage that the alkali atom or atom group combines with the anion of the stronger acid to form the corresponding soluble salt, which must then be transformed by devious methods into the corresponding hydroxides or carbonates if a recovery of the alkali is contemplated.

It is an object of the present invention to provide a method of separating terephthalic acid from aqueous solutions comprising substantial quantities of alkali terephthalates, which will not only produce good yields of pure terephthalic acid but will also combine the alkali radical in a form suitable for direct re-use in a process for the production of more terephthalic acid, involving the rearrangement of benzene carboxylates at elevated temperatures.

Other objects and advantages of the present invention will become apparent as the description thereof proceeds.

I have found that dialkali salts of terephthalic acid in aqeous solution can easily be divided into difiicultly soluble monoalkali salts and alkali bicarbonate by introducing carbon dioxide into the solution, and that the diflicultly soluble monoalkali salts of terephthalic acid can be {hydrolyzed with water into free terephthalic acid and dialkali salts of'terephthalic acid. The free terephthalic dioxide at the prevailing temperature.

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acid separates out as a solid, while the dialkali terephthalate remains in solution.

Of particular interest as alkalies are sodium, potassium and ammonium. For purposes of illustration, the invention will be described only in conjunction with the potassium salts, but the principles can be analogously applied to other alkali terephthalate compounds.

The following reactions form the basis of the process in accordance with the invention:

If the aqueous solutions used as the starting materials are not entirely clear, they are advantageously freed from insoluble and other undesirable substances by filtration or any other purification process; for example, by treatment with activated charcoal. The concentration of dipotassinm terephthalate may vary within wide limits. The

minimum concentration of dipotassium salt theoretically required is theequivalent amount of monopotassium terephthalate which can be dissolved under the reaction conditions in the amount of water used. A saturated aqueous solution of monopotassium terephthalate at about 20 C. contains from 0.05-0.06% by weight of this salt. However, in general the process is carried out at consider ably higher concentrations; for example, at concentrations from 1-10% by weight of the dipotassium salt based on the weight of the entire solution. It is also possible to work with a fully saturated dipotassium terephthalate solution; the saturation concentration depends substantially upon the working temperature. At 20 C. the saturation concentration is, for example, about 15% by Weight. The solution may even contain suspended undissolved dipotassium terephthalate, because it dissolves in the course of the reaction and reacts to form potassium bicarbonate and monopotassium terephthalate.

1 Since the presence of dissolved carbon dioxide in the solution is a prerequisite for theperformlance of the process, it is advantageous to work at moderately elevated temperatures; for example, below 50 C. and preferably below 30 C. The working temperature may, however, be considerably lower and may even be as low as the freezing point of the solution. It is, however, preferred to work at temperatures above 0 C. The introduction of carbon dioxide into the aqueous solution may be accomplished in any desired manner.

It is possible to work at atmospheric pressures or the carbon dioxide may be introduced at a pressure which may be as high as the vapor pressure of pure carbon Shortly after the carbon dioxide is introduced, the monopotassium terephthalate begins to precipitate. This precipitation may, if desired, be carried out fractionally, for example for the purpose of purification. The introduction of the.

ple when pressure vessels are used, the theoretical amount of carbon dioxide is sulncient to bring about a virtually quantitative transformation of, the dipotassium tereph If the process thalate into monopota'ssium terephthalate and potassium bicarbonate. A small'excess which is suflicient to maintain the pressure is usually employed.

After the monopotassiurn,terephthalate has been precipit'ated and filtered off, the"-filtra te contains small amounts of dipotassium terephthalate in addition to potassium bicarbonate. If it is also desired to separate these 'dissolved quantities of dipotassium terephthalate, thesblution-is concentrated by evaporation and the'di'ssolved sa'lt is again precipitated with carbon dioxidafro'm the concentrated solution; 7

' When treating sodium terep'hthalate solutions in an analogous fashion, the first precipitation fractions are obtained free from bicarbonate. If, on the basis of the existing concentration conditions, the precipitation of sodium bicarbonate can be expected, it is advantageous to operate'at somewhat higher temperatures, for example at 30-50 C., so that the sodium bicarbonate remains in solution.

The potassium bicarbonate solution thus obtained contains approximate one-half of the potassium in the original solution. In that form it may directly be used, for example in the preparation of potassium benzoate or phthalate, by dissolving orthophthalic acid or ben zoic acid therein and evaporating the solution to dryness. The dry salts may then be employed as starting rnat e lials for the production of terephthalic acid by rearrangement atflelevated temperatures, as above set forth. It isfhowever, also possible to drive on carbon dioxide by heatingthe solution and thereby to transform the potassiumbicarbonate" into potassium carbonate, which may then be transformed into potassium hydroxide by reactionwith calcium hydroxide. As in the case of the potassium bicarbonate solution, the potassium carbonate or potassium hydroxide solution can also be used for dissolving ortho'phthalic acid or ,benzoic acid or for other suitable purposes.

In order to recover the second half of the potassium contained in the precipitated monopotas siur'n terephthalate, the precipitate is suspended in water and hydr'o lyzed to form terephthalic acid and potassium terephthalate. Because of its low solubility, the te'rephthalic acid precipitates out. In this step the reaction conditions depend largely upon the concentration of the solution in which the hydrolysis of the monopotassium terephthalate is carried out. The term concentration relates to the entire amount ofsuspended monopotassium terephthalate in contact with the water, without taking into consideration the small amounts of dissolved monopotassium terephthalate which may possibly be present. The minimum amount of monopotassium terephthalate to be used with any given amount of water must be such that the amount of terephthalic acid formed in accordance with reaction (2) above is larger than the quantity which can be dissolved by the water present in the system. Since the solubility of terephthalic acid in water is very small (0.00l5% at C.; 1.8% at 200 C.), this separation step is carried out at considerably higher concentrations. However, in this reaction phase, it is advantageous to work in a moderately concentrated solution, because the hydrolysis of monopotassium terephthalate is enhancedunder these conditions; it proceeds more rapidly in more dilute solutions and at lower temperatures (about 50 C.) than in more highly concentrated solutions. On the other hand, if this phase is carried out in more dilute solutions and at more elevated temperatures, a relativelylarge part of the treated monopotassium terephtha'late remains-in solution inthe form of dipotassiuni terephthalate. It is therefore advantageous to operate at concentrations from 15% by' w'eight of monopotassium terephthalate and at the boiling point of the solution. In general, atmospheric pressure is used; only at higher concentrations, for example in the range of'5-10% by weight, is it'advan- '4 tageous to use elevated pressures, which may range up to the critical pressure"'of waterunder' the prevailing conditions. f

If, in place of potassium salts, the corresponding ammonium or sodium salts are treated, the working conditions may be correspondingly varied to take into account the different solubilities of the ammonium or"so'-- few of the benzene tricarboxylic acids, as well.as the benzdie acid, also form d ifiiculty" s'o'luble acid salts which may precipitate" after the complete pre'cipitation' of the monoterephthalate; and possibly even" after the evaporation'of' the solution; Orthophthalic acid rernains' insolurieninthe form or thedipotassiurn" salt. Themoth'er liquors obtained in the various'reaction phases fo'r example, by '"eva'porating'"the"alkali bi'carbonate's'oluti'on and the carbon dioxide "formed by di'sso'lving'ortho phthalic acid or'benzo'ic acid in alkali carbonate or alkali bicarbonate sa1'ts'-ar'e advantageously recycled into the process. I I

The dissociation 'of dialkali terephthalat'es by the carbon "dioxide into' monoalkali tereph'thalates and potassium bicarbonate was not predictable, because terephthalic acid is' 'a much stronger organic -a'c'icl (K =2.8- 10- K 4'.0- l0 than the relatively"weak carbon dioxide (K 4'.3-1'0- However, also the second process step-'na"mely, the" hydrolysis of the acid alkali terephthalate' into terephthalic acid and dialkali tereplith'alate-was not predictable. Itcan be regarded as a special advantage of the process according to the invention that" the recovery of the free te'replithalic'j acid from the reaction product, its 'putification and the recovery of the alkali'can be'ca'rrie'd out in a single continuous procedure, and that all of the reagentsfsuch as the alkali and'the carbon dioxide, canbe re'cycled 'into the process.

in carrying'out the proce'ss'on an"industrial scale,

.it is sometimes necessary to make" an analyticaldeten minatidn of'the am'oant' of 'monoor dialkali tere'phthala'tes, or mixtures thereof, in thes'olutions. The mono= alkali tere'phthalate present in an aqueous solution can betitr'ated' with alkali" hydroxide in the same way as a,

i monoba'sic' weak acid in the'presen'ce of phenolphthalein.

In order to'determ'ine' the presence of dialkali terephthala'te in'addition to monoalkali tereplithalate in aqueous solution, the' total amount of tere'phthalic acid is precipitated at the boiling point of the solution with'a measured excess of a standard solution of a strong acid, such as sulfuric acid, then boiled for two-minutes; and filtered at rcem ternpei-atnre through a glass fr'itva'cuuni filter; the filter residue is washed with water and the excess acid inthe filtrate is back-titrated w ith alkali hydroxide in the presence of methyl orange. Theportion of acid equivalent to the dialkali terephthalate is thenequal to thediiference between-the amount of acid used up for the neutralization of the total amount of terephthalic acid and the amountof alkali-hydroxide used to: neutralize the monoalk'a'literephthalate.

The following examples will further illustrate the p'rescut-invention and enable-others skilled in thea'rt'to understand "my invention more 1 completely. However, these examples 'are given only-for the purpose of inns:- tration-and"explanation,and I do not wish to be limited to' the exact-materials and conditions recited therein.

Example I From a cold saturated aqueous solution of dipotassium 1st fraction- 8.5 gm.; acid No.=262/264; 19.44% K 2nd fraction=5.0 gm.; acid No.=262.5/265; 19.38% K 3rd fraction=4.2 gm.; acid No.=224.5/221.5; 22.30% K 4th fraction=5.6 gm.;; acid No.=271/268; 19.48% K Since further introduction of carbon dioxide into the filtrate did not precipitate any more monopotassium terephthalate, the solution was concentrated by evaporation from 150 cc. to about 55 cc. Upon introducing further carbon dioxide into the concentrated solution, a fifth fraction was precipitated weighing 2.0 gm.; acid No.=243/247. After evaporating the filtrate of this precipitation fraction by boiling until the volume was reduced to 18 cc. and then introducing additional carbon dioxide, a sixth fraction weighing 0.5 gm., acid No.=255/ 255, was precipitated. The 18.0 cc. of mother liquor were found to contain 0.34 gm. terephthalic acid, 5.9 gm. potassium and 7.0 gm. carbonate.

Example II 5.0 kg. of technically pure (98.3%) terephthalic acid and 6.8 kg. extra pure potassium hydroxide in the form of a 50% aqueous solution were dissolved in 26.6 kg. water. This solution was filtered but only very small, virtually unweighable amounts of a slimy substance were separated out. By passing carbon dioxide into the clear solution, monopotassium terephthalate was fractionally precipitated out. Each precipitate was filtered with a strong vacuum suction and the filter cake was washed with a small amount of water. The following fractions were obtained: 1

1st fraction: 2.035 kg.; acid No.=275/277 2nd fraction: 1.135 kg.; acid No.=274/274 3rd fraction: 1.245 kg.; acid No.= 269/272 By evaporating the mother liquor to 10 liters, introducing carbon dioxide into the evaporated solution, filtering off the precipitate, evaporating the filtrate to 6 liters, again introducing carbon dioxide and filtering off the precipitate formed thereby, the following two additional fractions were obtained: 1 1

4th fraction: 1.020 kg.; acid No.=270/274 5th fraction: 0.555 kg; acid No.=2 69.5/272 The filtrate of the fifth fraction still contained 57 gm. of terephthalic acid and 2.36 kg. potassium.

Example III Example IV The starting material was the reaction mixture obtained by subjecting dipotassium orthophthalate to a thermal rearrangement reaction at temperatures above 340 C. in

an inert atmosphere. The reaction product was dissolved in water, and the solution was freed by filtration from undissolved components, such as carbon-containing dissociation products. The filtered solution contained the following principal components;

10.6 percent K 3.5 percent G0,,

9.9 percent terephthalic acid 0.5 percent benzoic acid 0.35 percent trimesitinic acid accompanied by stirring; the pressure was maintained at about 2 kg. per square centimeter. 6.34 kg. of monopotassium terephthalate were precipitated thereby. After filtering this solution and evaporating the filtrate to about one-third of its original volume, it was again treated with carbon dioxide whereby 1.96 kg. of monopotassium terephthalate precipitated; the precipitate was filtered offs After concentrating the filtrate by evaporation and repeating the carbon dioxide treatment, 0.508 kg. of monopotassium terephthalate precipitated out. The separated fractions of monopotassium terephthalate were repeatedly washed with small amounts of water and the various wash water fractions were combined with the mother liquors. The total amount of 8.808kg. monopotassium terephthalate represented 96.4% of the total of 9.15 kg. of acid potassium terephthalate which could be theoretically precipitated according to the analytical data. A determination of the terephthalic acid content of the mother liquor revealed that it contained 246 gm. terephthalic acid=3.4% of the terephthalic acid in the starting material.

After crystallizing the acid potassium terephthalate from the solution, two separate solutions were obtained: one weighing 33.6 kg. with 15.44% K, and the other weighing 7.1 kg. with 15.15% K. This corresponds to a total amount of potassium of 6.27 kg. If the 1.68 kg. of potassium precipitated in the form of the acid salt are added thereto, a total amount of 7.95 kg. potassium is obtained, which corresponds to the previously analytically determined amount of 7.96 kg. potassium. This solution was then used for again dissolving orthophthalic acid anhydride therein, and thereby forming a new batch of starting material for thermal rearrangement into dipotassium terephthalate, as above set forth.

Example V tained. The filtrate contained 407 gm. acid potassium terephthalate dissolved therein and 8.36 kg. dipotassium terephthalate. The pH of the solution changed during the reaction from 5.0 to 7.0. The mother liquor was subsequently used to dissolve raw dipotassium orthophthalate.

Example VI Carbon dioxide was introduced into a cold saturated aqueous solution containing 31.6 gm. disodium terephthalate in 250 cc. of solution. 3.0 gm. monosodium terephthalate (acid No. 299/302) were precipitated. Thereafter, the solution was evaporated until disodium Example VII 25 gm. of pure terephthalicacid were suspended in 400 gm. water and dissolved with 21 gm. of aqueous ammonia solution (density=0.910) until the solution became clear. 'By alternately precipitating monoammonium terephthalatewiththe aid of carbon dioxide introduced into the solution and thereafter evaporating the:

filtereds'olution until.theprecipittttion of 'the diaminoniumsaltbegan, about 87 of terephthalic acid: were isolated from. the solution in. the form of its' monoammoniurn: salt. Theindividual; precipitates had" acid :nu'm-- bersranging bet-ween:340-and 390. In this case, there" fore, the precipitated acid sa lt also contained small amounts 015 freeterephthalic acid.

While I- have given examples of specific embodiments of my invention, I wish it to be understood that thelpresent invention is not limited to these embodiments and that various changes and modifications may be made without departing from the" spirit of the invention or-the scope of the appended claims.

. I claim:

1. The proces's of separating ter'ephthalic' acid fromanv aqueous solution containing from about 1% to about 15% by Weight of a dialkali salt of terephthalic acid, which comprises introducing gaseous carbon dioxidei'nto the aqueous solution at temperaturesfrom' about C. to about 50 C. until a substantial quantity of the corresponding monoalkali salt of terephthalic acid is formed and precipitated from the solution, separating the monoalkali salt from the solution, forming an aqueous solution containing from about 1 to about of the monoalkali salt, hydrolyzing the monoalkali salt in water at elevated temperatures until substantially all of the monoalkali salt is converted into the corresponding dialkali salt of t'erephthalicacidand terephthalic acid and separating the terephthalic' acid from the reaction mixture.

2. The process of separating terephthalic acid froman aqueous solutioncontaining from about 1% toabout by weight of-a di-salt of terephthalic' acid: selected from the group consisting of disodium, dipotassium and diammoniurn salts, which comprises introducing-carbon dioxide into the aqueous solution 'at temperatures from about '0 C. to about50 Cruntil a substantial-quantity of the'corresponding mono-salt of ter'eplith'alic acid: is formed and precipitated from the solution, separating-the mono-salt from the solution, forming an aqueoussolution containing from' 'abo'ut 1% to about 10% of the mono-salts, hydrolyzing the mono-salt until substantially all of the mono-salt is converted into the corresponding di-salt of terephthalic acid and-terephthalic acid, and separating the terephthalicacid from the reaction mixture.

3. The process of separating terephthalic acid from an aqueous solution containing fro'm' about 1% to about 15% by weight of dipotassiurn terephthalate, which'comprises introducing gaseous carbondioxide into the solution-at temperatures'from about 0- C. to about 50 C. until a 8. substantial quantity ofi" monopotassium terephthalate is formed and precipitated from the solution, forming an aqueous solution con'taining from about 1% to about 10% of the monoalkali salt, hydrolyzing the mOnopo'tassiurn 'terephth alate in water at elevated temperatures until-substantially all of them'onopotassium salt is converted into dipotassium terephthalate and terephthalic acid,- and separati'ng the terephthalic acid from the reaction mixture.

4. The. process of separating terephthalic acid from an aqueous solution containing from about 1% to about 15% by weight of disodium terephthalate, whichcomprises introducing gaseous carbon dioxide into the solution at temperatures from-abQm O C. to'about 50 C.

' until a substantial quantity of monosodium terephthalate is formed and-"precipitated from the solution, separating the monosodium salt from the solution, forming anaqueous solution containing't-rom about 1% toabout 10% of the monoalkali salts, hydrolyzing the monosodium salt in-water'at elevated temperatures until substantially 'all of the monosodium salt is converted into disodium terephthalate and terephthalic acid, and separating the terephthalic acid from the reaction mixture.

5. The process of separating terephthalic acid from an aqueous solution containing from about 1% to about 15% by weight of diainmonium terephthalate, which comprises introducing. gaseous carbon dioxide into the solution at temperatures from about 0 C. to about 50 C. until-a substantial quantity of monoammonium terephthalate isformed and precipitated from the solution, separating the monoammonium salt from the solution, forming an aqueous solution containing from about 1 to about 10% of the monoalkali salt, hydrolyzing the monoammonium salt inwater at elevated temperatures until substantially all of the monoammonium salt is conv'erted into' diammonium terephthalate and terephthalic acid, andseparating the t'er'ephthalic acid from the reactio'n mixture.

6. The process ofproducing monoalkali salts of terephthalic acid from an aqueous solution containing from about 10% to'about 15% by weight of a dialkali salt of terephthalic acid, which comprises introducing carbon dioxide into the solution at temperatures from about 0 C. toabout 50'C. until a substantial quantity of the corresponding monoalkali salt of terephthalic acid is formed and precipitated from the solution, and separating the precipitated monoalkali salt from the solution.

7. The process of-producing terephthalic acid anda dialkali salt of terephthalic acid, which comprises hydrolyzing a monoalkali salt of terephthalic' acid in water-wherein the concentration of the monoalkali salt is from about'l to about 10% by weight at elevated temperatures, separatingthe insoluble terephthalic' acid formed thereby from'the aqueous inedium,'an'd separating the dialkali terephthalate simultaneously formed and dissolved in the aqueous medium'b'y evaporating the aqueous medium.

References Cited in the file of. this patent UNITED-STATES PATENTS 1,822,016 Daniels Sept. 8, 1931 2,697,723 (iarlstoniet a1.khan. Dec. 21, 1954 2,789,134 Nelsonef a1; Apr. 16, 1957 

1. THE PROCESS OF SEPARATING TEREPHTHALIC ACID FROM AN AQUEOUS SOLUTION CONTAINING FROM ABOUT 1% TO ABOUT 15% BY WEIGHT OF A DIALKALI SALT OF TEREPHTHALIC ACID, WHICH COMPRISES INTRODUCING GASEOUS CARBON DIOXIDE INTO THE AQUEOUS SOLUTION AT TEMPERATURES FROM ABOUT 0*C. TO ABOUT 50$C. UNTIL A SUBSTANTIAL QUANTITY OF THE CORRESPONDING MONOALKALI SALT OF TEREPHTHALIC ACID IS FORMED AND PRECIPITATED FROM THE SOLUTION, SEPARATING THE MONOALKALI SALT FROM THE SOLUTION, FORMING AN AQUEOUS SOLUTION CONTAINING FROM ABOUT 1 TO ABOUT 10% OF THE MONOALKALI SALT, HYDROLYZING THE MONOALKALI SALT IN WATER AT ELEVATED TEMPERATURES UNTIL SUBSTIANTIALLY ALL OF THE MONOALKALI SALT IS CONVERTED INTO THE CORRESPONDING DIALKALI SALT OF TEREPHTHALIC ACID AND TEREPHTHALIC ACID, AND SEPARATING THE TEREPHTHALIC ACID FROM THE REACTION MIXTURE. 